1. Field of Use
The present invention relates generally to an underwater communications system, and its methods of use, and more particularly to an underwater communications system that uses electromagnetic propagation and magnetic induction transmission, and optimizes the distance which can be achieved by digital transmission of information.
2. Description of the Related Art
Various underwater communication systems are known. One of the most common is based on acoustic techniques. A problem with such systems is that they are degraded by noise and interference from a number of sources. They are also subject to multi-path effects and in some environments are virtually unusable. Other underwater communication systems use radio links, e.g. extreme low frequency electromagnetic signals, usually for long-range communications between a surface station and a submerged vessel. These systems typically operate in the far field using physically large electric field coupled antennas and support data rates up to a few bits per second.
WO01/95529 describes an underwater communications system that uses electromagnetic signal transmission. This system has a transmitter and a receiver, each having a metallic, magnetic coupled aerial surrounded by a waterproof electrically insulating material. Use of electrically insulated magnetic coupled antennas in the system of WO01/95529 provides various advantages. This is because magnetically coupled antennas launch a predominantly magnetic field. A similar arrangement is described in GB2163029. Whilst the communications systems of WO01/95529 and GB2163029 have some technical advantages over more conventional acoustic or radio link systems, the functionality described is limited, and for many practical applications the available bandwidth is highly restrictive, as is distance over which data can be transmitted.
Magnetic antennas formed by a wire loop, coil or similar arrangements create both magnetic and electromagnetic fields. The magnetic or magneto-inductive field is generally considered to comprise two components of different magnitude that, along with other factors, attenuate with distance (r), at rates proportional to 1/r2 and 1/r3 respectively. Together they are often termed the near field components. The electromagnetic field has a still different magnitude and, along with other factors, attenuates with distance at a rate proportional to 1/r. It is often termed the far field or propagating component.
Signals based on electrical and magnetic fields are rapidly attenuated in water due to its partially electrically conductive nature. Seawater is more conductive than fresh water and produces higher attenuation. Propagating radio or electromagnetic waves are a result of an interaction between the electric and magnetic fields. The high conductivity of seawater attenuates the electric field. Water has a magnetic permeability close to that of free space so that a purely magnetic field is relatively unaffected by this medium. However, for propagating electromagnetic waves the energy is continually cycling between magnetic and electric field and this results in attenuation of propagating waves due to conduction losses.
The attenuation losses, the bandwidth restrictions and the limited distances over which data can be transmitted all pose significant practical problems for underwater communications.
Existing methods of acoustic communication are inherently restricted in the distance they can achieve at effective data rates. This is particularly true where the signal reaches a receiver by multiple paths (reflections occurring from an irregular sea floor, the sea surface, the coastline, nearby objects and the like), and when the sound wave path exhibits discontinuities in its properties (wave wash, bubbles in the water, changes in water density due to salinity variations). Little is known which can lessen these difficulties. The existing art of electromagnetic communication under water fails to recognize measures that can be taken to maximize the distance and/or useful information rate which can be achieved by adapting the devices sourcing and using the information so that more effective signal frequencies can be adopted.